1. Field
The present disclosure relates to a composite separation membrane structure for a gas sensor, a gas sensor apparatus including the same, and a method and an apparatus for measuring gas concentration using the same. More particularly, the disclosure relates to a composite separation membrane structure for a gas sensor, a gas sensor apparatus including the same, and a method and an apparatus for measuring gas concentration using the same, which allow for real-time monitoring of degradation of insulating oil used in power equipments such as a power transformer.
2. Description of the Related Art
Power transformer is a very important power supplying equipment. In order to prevent any abrupt malfunction of power equipments and resulting blackout and economically operate the power equipments and predict their service life, a condition monitoring technology allowing for monitoring of the internal faults of the power transformer so as to prevent dangerous accidents is necessary.
The condition monitoring technologies currently used for diagnosis of internal faults that may occur in the power transformer insulating oil may include gas-in-oil analysis, measurement of power factor and water content of insulating oil, measurement of partial discharge, low-pressure surge test, and so forth.
Among the methods, the gas-in-oil analysis method may allow for detection of the degradation of insulating oil inside the power transformer as the degradation proceeds. Further, since the gas-in-oil method is technically reliable and easy for real-time application, it is employed most frequently.
The gas-in-oil analysis method will be described in more detail.
A power transformer is subject to a constant heat as an electrical coil is used in the power transformer. A local electrical breakdown inside the power transformer may result in a partial arc discharge of high temperature.
Accompanied by the situation, the hydrocarbon-based insulating oil may be thermally decomposed to generate gases such as hydrogen (H2), methane (CH4), acetylene (C2H2), ethylene (C2H4), etc. In particular, when there is an insulating material such as insulating paper, pressboard, Bakelite etc. around the heated portion, gases such as carbon monoxide (CO) or carbon dioxide (CO2) may also be generated.
For reference, among the gases, those such as hydrogen, methane, acetylene, ethylene, ethane, propane, etc. are highly combustible, and thus are very important components in terms of diagnosis and safety management of the power transformer.
Since most of these gases are soluble in the insulating oil, it is possible to diagnose whether there is a fault in the power transformer and what kind of fault occurs where in the power transformer by extracting the gases and analyzing them quantitatively/qualitatively.
For analysis of the gases included in the insulating oil, a method of extracting an insulating oil sample from an operating power transformer, bringing the insulating oil in a laboratory, extracting gases from the insulating oil and analyzing them with a gas chromatography is generally employed.
However, this laboratory based analysis has a low reliability resulting from the human error factors that may occur during the sampling, and the analysis of results requires a lot of time as well.
Thus, attempts have been made recently to install a direct real-time measurement apparatus inside or outside of the power transformer to allow for continuous measurement and monitoring.
For example, a microfiltration filter or an ultrafiltration filter device that can filter gases in the insulating oil from the insulating oil medium may be used in order to sample gases dissolved in the insulating oil from the inside of the power transformer.
According to the study of the inventors, however, such gas sampling is made possible by applying a negative pressure using a vacuum pump at the rear side of the filter. This is because the gases dissolved in the insulating oil are extracted through the filter device across which a pressure difference is heavily applied.
However, considering the relatively short service life of the vacuum pump, this method does not seem so practical for the real-time condition monitoring technology of the power transformer whose service life should be as long as about 10-30 years.
As for another method for analysis of the gases dissolved in the insulating oil, the patent document 1 discloses an apparatus and a method for monitoring faults inside a power transformer where the faults are diagnosed by extracting and separating the gases dissolved in the insulating oil using an oil/gas separation membrane and detecting the total concentration of the gases in the insulating oil using an electrochemical gas sensor.
However, according to the study of the inventors, the patent document 1 does not specifically describe how to make the separation membrane and effects resulting from the separation membrane.
Meanwhile, a semiconductor gas sensor including a metal oxide such as tin oxide, tungsten oxide, zirconium oxide, etc. is generally known to be capable of quantitatively measuring the concentration of gas components since their electrical properties may change in response to the gas presented. The semiconductor gas sensors are frequently used for measuring pollutant gases in the air.
However, when the semiconductor gas sensor comes into direct contact with the insulating oil or comes into direct or indirect contact with oil fume produced from the insulating oil in order that the degradation of the insulating oil in the power transformer is measured, the pollutants are highly likely to contaminate the surface of the semiconductor gas sensor and to that end lead to wrong measurements.
The patent documents 2, 3 and 4 disclose an apparatus and a method for analyzing gases in insulating oil by detecting the concentration of the gases in the oil using a commercially available semiconductor gas sensor, and further disclose that a separation membrane may be selectively used for extraction and separation of the gases dissolved in the insulating oil.
However, according to the study of the inventors, the patent documents do not specifically describe how to make the separation membrane and effects resulting from the separation membrane either.
In addition, according to the study of the inventors, in the patent document 2, concentrations of a plurality of individual gases are determined sequentially, for example in a manner that the concentration of hydrogen is determined first, and then the concentration of carbon monoxide is determined. However, since this method requires special sensors capable of sensing specific gas, for example hydrogen, it is not appropriate for the generally-used semiconductor gas sensor to be used in the method.
Meanwhile, the patent document 5 presents the use of a porous PTFE polymer material having a thickness of about 1-1,000 μm and a porosity of about 5-99% for separation of gases dissolved in a fluid, and describes specific examples of stretching of the surface of the polymer material, solvent extracting or casting to obtain a porous surface.
Further, the patent document 6 presents porous polymer materials (poly(tetrafluoroethylene) and poly(tetrafluoroethylene-cohexafluoropropylene)) having a thickness of about 1-5 mm and a pore size of about 0.001-0.1 mm for extraction and separation of gases dissolved in oil medium, and describes specific examples of forming pores on the surface of an oil/gas separation membrane by bombarding the polymer material with 13.56 MHz radio frequency (RF)-type argon gas or nitrogen gas laser having an output power of about 100-500 W and a pressure of about 0.5-50 Pa for about 10-30 minutes.
However, according to the study of the inventors, even in the methods disclosed in those patent documents, it is highly likely that pressure difference across the separation membrane is inevitable. Furthermore, the processing of the separation membrane is complicated and requires a special technique.